1. Field of the Invention
The present invention relates to an LED emission wavelength estimation method, an image reading apparatus, and a multifunction printer apparatus. Particularly, the present invention relates to an LED emission wavelength estimation method of estimating, for example, the emission wavelengths of the LEDs of a CIS sensor to be used to optically read an image original, an image reading apparatus, and a multifunction printer apparatus.
2. Description of the Related Art
A color scanner is known as an image reading apparatus which reads an image by switching light of different light emission wavelengths. Such a color scanner has a linear light source and an image sensor provided on a carriage movable in a predetermined direction. The light source includes LEDs capable of irradiating light emission wavelengths corresponding to three primary colors of light, that is, red (R), green (G), and blue (B). The carriage is moved in a direction (sub-scanning direction) perpendicular to the elongated direction (main scanning direction) of the linear light source. The liner image sensor receives reflected light obtained by irradiating an image original with light and reads the image original. To read the image original, a scanning read method is employed.
In the scanning read method, an original is read by switching three LEDs serving as a light source while conveying a CIS (Contact Image Sensor) unit in the sub-scanning direction. More specifically, the R component data of a color image is obtained by lighting a red LED. Next, the G component data is obtained by lighting a green LED. Finally, the B component data is obtained by lighting a blue LED. Image data of one line is obtained in one red, green, and blue LED lighting cycle. Image data of one page of the image original is obtained by repeating the lighting cycle while conveying the CIS unit in the sub-scanning direction.
The read image data then undergoes color correction processing by an image processing unit. The image data is thus corrected to values corresponding to a desired tone so that final read image data is obtained.
Generally, in color correction of such a color scanner, the colorimetric values (for example, CIE-L*a*b*) of color patches on the image original and the RGB output signals of the device that has read the color patches are made to match RGB values corresponding to the colorimetric values. This processing is executed using known arithmetic processing or a one- to three-dimensional lookup table (LUT) in a predetermined color space such as sRGB. Especially, to make colors nonlinearly match in a color space, a three-dimensional LUT is necessary in many cases.
Even a color patch having a matched colorimetric value changes the values of device RGB output signals between, for example, a silver halide photo and an inkjet photo printed on glossy paper because of the difference in spectral reflectivity generated by different color materials. This is known as a problem of metamerism. Techniques of coping with the problem of metamerism are disclosed in, for example, “Fuji Xerox Technical Report No. 7, 1992” p. 30, and Japanese Patent Laid-Open Nos. 6-197218 and 2002-094814.
“Fuji Xerox Technical Report No. 7, 1992 p. 30” discloses a technique of switching two sets of color correction parameters for printing and photographic paper in accordance with a user's original designation. Japanese Patent Laid-Open No. 6-197218 discloses a technique of automatically determining the original type based on the histogram of an image. Japanese Patent Laid-Open No. 2002-094814 discloses a technique of automatically determining the color material type by reading an image original under different read conditions.
The emission wavelengths of the LEDs of the light source of the conventional color scanner generally have a variation of about ±10 nm. If the emission wavelengths of the LEDs vary, the values of the above-described device RGB output signals change. Hence, even though color correction of making the values match the RGB values in a predetermined color space is performed, color shift still occurs. In addition, since color change by metamerism associated with the change of color materials also exhibits another behavior, the techniques of coping with the metamerism disclosed in the above-described references do not function well.
To cope with the variation in the wavelength of the color scanner light source, Japanese Patent Laid-Open No. 2006-229351 discloses a technique of reading a chromatic printer correction pattern by a scanner and correcting the tone characteristic of the reading apparatus. However, this technique does not measure the light source wavelength itself but detects only the variation in the tone characteristic of the reading apparatus by the wavelength variation of the light source using a patch of complementary color for the light source. Hence, this technique is not applicable to the problem of metamerism or the above-described color correction of making the colors match a color space using a three-dimensional LUT.
In a multifunction printer apparatus (MFP apparatus) that integrates a color scanner and a color printer, the printed colors of the printer have significant individual variations or time-rate changes.